1. Field of the Invention
The present invention relates to a process for producing a buried wave guide device which is used for branching or coupling of a light in fields such as optical communication and the like.
2. Prior Art
A compared with optical circuits of microoptics type produced by combination with a micro lens, prism or optical fiber, optical circuits of wave guide type produced by allowing high refractive index ion(s) to diffuse into a portion of a substrate to form a wave guide or a lens in the substrate, are advantageous in that they require no alignment and are resistant to vibration because the wave guide and the lens are formed in one substrate. Accordingly, the wave guide type optical circuits are finding wide applications. As one of the wave guide type optical circuits, there is a branching circuit sing a wave guide (a slab wave guide), such as presented at the 4th Optical Meeting on Gradient Index Optical Imaging System (Jul. 4-5, 1983, Kobe) by K. Sano et al. The wave guide device 3 presented has a structure as shown in FIG. 5, in which a wave guide 4 is formed in the vicinity of the upper surface of a substrate, fiber arrays 5 and 6 are provided at the both ends of the wave guide 4 and thereby an optical signal emitted from one optical fiber at one end of the wave guide 4 can be divided into a plurality of optical fibers at the other end of the wave guide 4. This wave guide 3 has hitherto been produced by the following two steps. That is, a plate-like glass containing large amounts of alkali ion(s) (e.g. Na.sup.+, K.sup.+) is produced, a metal film is formed on the four side surfaces and the bottom surface of the plate-like glass according to a known method to mask the surfaces, then the plate-like glass is immersed in a molten salt containing compound(s) of ion(s) (e.g. Ag.sup.+, Tl.sup.+, Li.sup.+) capable of giving high refractive index to the glass (said ion(s) being hereinafter referred to as "high refractive index ion(s)"), to allow these ion(s) to diffuse into the plate-like glass (the first step); subsequently, the plate-like glass is immersed in a molten salt containing compound(s) of ion(s) (e.g. Na.sup.+, K.sup.+ ) capable of giving low refractive index to the glass (said ion(s) being hereinafter referred to as "low refractive index ion(s)"), to ion exchange, at the peripheral portion of the glass, the ion(s) of high refractive index with the ion(s) of low refractive index (the second step); the first and second steps are effected while applying, if necessary, an electric field, whereby is obtained a buried wave guide device whose refractive index is maximum at the center of the wave guide and decreases gradually from the center to the peripheral portion of the wave guide. The above two-step method is explained further by referring to FIG. 6 showing the changes at each step, of the concentrations of ions in the plate-like glass. A plate-like starting glass having a uniform concentration 7a of alkali ion from the surface throughout the thickness d.sub.0 as shown in FIG. 6(A) is masked at the four side surfaces and the bottom surface, then the glass is immersed in a molten salt containing compound(s) of high refractive index ion(s), whereby ion exchange takes place between the alkali ion(s) and the high refractive index ion(s) in the portion of the glass ranging from the upper surface to a desired depth and there is obtained a plate-like glass in which the alkali ion(s) concentration 8a and the high refractive index ion(s) concentration 8b vary from the surface toward the depth direction as shown in FIG. 6(B) (the first step); the plate-like glass is then immersed in a molten salt containing compound(s) of low refractive index ion(s), whereby ion exchange takes place between the high refractive index ion(s) and the low refractive index ion(s) in the vicinity of the upper surface of the glass and there is obtained a buried wave guide device in which the alkali ion(s) concentration 9a, the high refractive index ion(s) concentration 9b and the low refractive index ion(s) concentration 9c vary from the upper surface toward the depth direction as shown in FIG. 6(C) and accordingly whose refractive index is maximum at a certain depth from the upper surface and gradually decreases from the depth to the peripheral portion (the second step).
As stated above, the conventional process for producing a buried wave guide device requires the two steps, i.e., the first step of allowing high refractive index ion(s) to diffuse into a plate-like starting glass and the second step of subjecting the high refractive index ion(s) already present in the glass to ion exchange with low refractive index ion(s). Therefore, the process is disadvantageous in requiring complex operation.
Hence, the object of the present invention is to provide a novel process for producing a buried wave guide device, which retains the advantages of the conventional diffusion and migration process (i.e., the resulting glass stably contains large amounts of high refractive index ion(s) and has a large difference in refractive index between the center and the peripheral portion of the glass), but is free from the complexity of the conventional diffusion and migration process and requires only one step of immersing a starting glass in a molten salt.